set_def might kill new_def in kill recursion caused by old_def being killed (#18)

1 files changed, 5 insertions, 3 deletions

diff --git a/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java b/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
index 8d698d1b..7cc9291c 100644
--- a/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
+++ b/chapter04/src/main/java/com/seaofnodes/simple/node/Node.java
@@ -193,14 +193,16 @@ public abstract class Node {
     Node set_def(int idx, Node new_def ) {
         Node old_def = in(idx);
         if( old_def == new_def ) return this; // No change
+        // If new def is not null, add the corresponding def->use edge
+        // This needs to happen before removing the old node's def->use edge as
+        // the new_def might get killed if the old node kills it recursively.
+        if( new_def != null )
+            new_def._outputs.add(this);
         if( old_def != null &&  // If the old def exists, remove a def->use edge
             old_def.delUse(this) ) // If we removed the last use, the old def is now dead
             old_def.kill();     // Kill old def
         // Set the new_def over the old (killed) edge
         _inputs.set(idx,new_def);
-        // If new def is not null, add the corresponding def->use edge
-        if( new_def != null )
-            new_def._outputs.add(this);
         // Return self for easy flow-coding
         return this;
     }